Our long-term objectives are to elucidate the physiological processes which lead to weakness and fatigue of skeletal muscle after human spinal cord injury (SCI), and to develop procedures by which functional impairment can be minimized or reversed. Specifically, we will use intraneural motor axon stimulation, surface stimulation and spike-triggered averaging to measure the EMG activity, force, speed and fatigue properties of human thenar single motor units in neurologically intact controlled muscles, and those that are completely paralyzed or partially paralyzed by SCI. F-waves will be monitored to assess motoneuron excitability changes. Motor unit counts will determine how many motor units remain excitable after SCI. Data obtained from all three methods will be compared to existing values from other human and animal muscles to assess whether reported differences are genuine, or whether these differences are mainly from the use of different methods of evaluation. Measurements of whole muscle contractile properties and activity will be used to determine how well these account for an aggregate of those recorded from single motor units, and the extent to which muscle weakness is due to disuse atrophy. The time course of changes in muscle strength, speed and fatiguability will be assessed periodically during the first year injury (acute to chronic transition). Additional evidence of muscle atrophy, non-contractile units and motor axon sprouting will be sought by measuring muscle fiber and motor unit sizes, as well as fiber type composition of paralyzed and control muscles, while the diameter and myelin thickness of the associated peripheral axons will be measured for evidence of fiber loss and/or regeneration. The extent to which morphological changes relate to functional deficits will be assessed. This information is essential for the design of better functional electrical stimulation (FES) techniques in which muscle atrophy, weakness, fatigue and spasticity are major unresolved problems. Most previous studies have focused on restoring the responses of paralyzed leg muscles for locomotion. But it is vital to know the properties of paralyzed intrinsic hand muscles, since the ability to grasp and perform self-care activities is as basic to human dignity and independence as standing and walking.